1. Field of the Invention
This invention relates to a method of, and an apparatus for, manufacturing a liquid-crystal display element, and more particularly, a liquid-crystal display element (panel) manufacturing method and apparatus which are designed to control surface potential in the step of rubbing an alignment film.
2. Description of the Related Art
Liquid-crystal display elements (panels) are widely used for display screens of portable-type personal computers and so forth. In recent years, they are demanded to have high quality such as high resolution, high contrast ratios and wide viewing angle. In particular, liquid-crystal display elements making use of a thin-film transistor (TFT) type active matrix (TFT-LCD) have performance comparable to conventional cathode-ray tubes in respect of picture quality. Also, liquid-crystal display elements are superior to cathode-ray tubes in respect of power consumption and space saving. Hence, in replacement of cathode-ray tubes, liquid-crystal display elements are often used in monitors for personal computers.
A process of manufacturing a liquid-crystal display element is briefly described below taking the case of a conventional manufacturing process for a transmission amorphous-silicon (a-Si) type color TFT-LCD. TFT-LCD manufacturing steps consist basically of a TFT formation step of fabricating thin-film transistors, metal wirings and so forth on a glass substrate, a color-filter formation step of fabricating color filters on another glass substrate, and an LCD assembly step of infilling liquid crystal to a space formed by setting these two resultant substrate members face to face.
In the TFT formation step, a series of steps of film formation, resist coating, exposure/development, etching, resist stripping and cleaning are repeated several times on gate wiring, interlaminar insulating film, a-Si film, pixel electrodes, protective film and so forth. Thus, a TFT substrate member is completed, which is then sent to the color-filter formation step.
In the color-filter formation step, after a black matrix has been formed, a series of steps of film formation, resist coating, exposure/development, etching, resist stripping and cleaning are repeated in regard to the three red, green and blue colors. After the three red, green and blue colors have been patterned, an over coat layer is further formed thereon to make the surface have a flatness. A transparent electrode film is finally formed, and then the color-filter substrate member obtained is sent to the LCD assembly step.
A common LCD assembly step for liquid-crystal display elements is outlined with reference to FIG. 1. An assembly step shown here is a step described keeping in mind an LCD assembly step for the a-Si type color TFT-LCD. Other transmission liquid-crystal display elements, reflection liquid-crystal display elements and projection liquid-crystal display elements are also passed through substantially the like LCD assembly step.
The TFT substrate member and the color-filter substrate member which have been formed as described above are sent to the LCD assembly step to carry out steps (substeps) of:
a surface cleaning step (S101) of cleaning the substrate member surface before an alignment film is printed thereon;
an alignment film printing step (S102) of printing an alignment film on the substrate member surface;
an alignment film baking step (S103) of baking the alignment film printed;
an alignment film surface rubbing step (S104) of rubbing the surface of the alignment film baked;
a post-rubbing cleaning step (S105) of cleaning the surface of the alignment film after rubbing;
a sealant patterning step (S106) of applying a sealant on the peripheral portions of the substrate members to make preparation for the formation of a liquid-crystal filling space between opposing substrate members;
a substrate-superposing step (S107) of forming the liquid-crystal filling space by superposing the color-filter substrate member on the TFT substrate member;
a liquid-crystal injection step (S108) of injecting a liquid crystal into the liquid-crystal filling space;
a liquid-crystal sealing step (S109) of closing an injection opening to enclose the liquid crystal hermetically in the filling space; and
a lighting inspection step (110) of lighting the TFT-LCD panel assembled, to make inspection. The TFT-LCD panel having been inspected by lighting is sent to a mounting step of mounting integrated circuits for driving and so forth.
In the step S104 rubbing process among the LCD assembly substeps, the alignment film formed on the TFT substrate member or on the color-filter substrate member is rubbed with a cloth in one direction (subjected to “rubbing”) to make treatment, in order to align liquid-crystal molecules orderly. As a result of this treatment, liquid-crystal molecules are arranged in the direction of rubbing treatment. Hence, the display performance and display quality of liquid-crystal display element depend greatly on the rubbing treatment.
In the rubbing treatment, a cylindrical roller around which a rubbing cloth such as nylon, Rayon or cotton cloth is kept attached is used. When the alignment film is subjected to rubbing, because of mechanical friction between the alignment film and the rubbing cloth the alignment film may partly come off, or the rubbing cloth fiber may partly be cut or may wear. In such a case, static electricity is generated upon friction at the same time the rubbing cloth fiber wears or is cut. As the result, wear pieces or cut pieces of the rubbing cloth fiber may adhere as foreign matter to the surface of the substrate member having been subjected to rubbing.
Such foreign matter eliminates the liquid crystal coated on the alignment film or disorders the alignment of liquid-crystal molecules to cause faulty display in pixels, and hence it must surely be removed. In order to remove the foreign matter, the surface of the substrate member is cleaned also after the rubbing. However, it is impossible to perfectly remove the foreign matter having adhered. The reason therefor is that the foreign matter having been removed may again adhere in the cleaning step subsequent to the rubbing.
In making the rubbing treatment of the substrate member, not only the alignment film but also the protective film and electrodes formed beneath the alignment film are simultaneously rubbed. The alignment film, the protective film and the electrodes individually have different coefficient of friction against the rubbing cloth, and hence the condition of wear of the rubbing cloth, the condition of adhesion of the foreign matter, the condition of electrification, and so forth also differ for each substance being rubbed.
To sum up the above problems, what must be settled is firstly the generation of static electricity at the time of rubbing, secondly the occurrence of foreign matter originating from the rubbing cloth at the time of rubbing, and thirdly the foreign matter again adhering to the substrate member in the post-rubbing cleaning step. Also, without limitation to the transmission a-Si type TFT-LCD described previously, these problems are likewise included in other transmission liquid-crystal display elements, reflection liquid-crystal display elements and projection liquid-crystal display elements.
To solve such problems, the following methods are proposed as disclosed in Japanese Patent Applications Laid-open No. 2-275926, No. 4-333824, No. 5-181139, No. 6-194664 and No. 8-334767.
In the disclosure of Japanese Patent Application Laid-open No. 2-275926, an apparatus which eliminates the static electricity generated by rubbing, is proposed, in which a rubbing roller is endowed with conductivity by using a rubbing cloth formed of a conductive material and using a conductive-material adhesive and at the same time the rubbing cloth is grounded via a resistor. By using this apparatus, any insulation breakdown of alignment film because of static charging caused by rubbing and any accumulation of static electricity can be kept from occurring. In this apparatus, however, it is impossible to keep the foreign matter from being caused by the wear of the rubbing cloth.
In the disclosure of Japanese Patent Application Laid-open No. 4-333824, a method is proposed in which a protective layer is provided on a substrate member having not been rubbing-treated, at this side of the end portion of an alignment film in its rubbing direction so that any foreign matter can be avoided adhering to the alignment film. This is a method of removing the foreign matter by bringing a substance other than the alignment film into contact with a rubbing cloth to make the substance catch the foreign matter adhering to the rubbing cloth. However, although the foreign matter adhering to the rubbing cloth can be removed at the position where the alignment film has not been rubbing-treated, its effect does extend up to the vicinity where the alignment film has been rubbing-treated.
In addition, its effect can not be expected as the effect of resetting the rubbing cloth at the position where the alignment film has not been started being subjected to rubbing, because the rubbing distance on the substrate member becomes longer as the substrate of a liquid-crystal display panel has a larger size. Also, in this method, too, it is impossible to keep the foreign matter from being caused by the wear of the rubbing cloth.
In the disclosure of Japanese Patent Application Laid-open No. 5-181139, a method is proposed in which a panel substrate member and a clean dummy substrate member are alternately subjected to rubbing so that the foreign matter originating from the alignment film, having adhered to the rubbing cloth, is drawn to adhere to the dummy substrate member to keep the coefficient of friction of the rubbing cloth from changing, to achieve uniform and stable alignment performance without regard to the number of alignment films to be treated. This method, however, is a method of removing the foreign matter originating from the alignment film, and is not a method of removing the foreign matter originating from the rubbing cloth. It is also impossible to keep the foreign matter from being caused by the wear of the rubbing cloth.
In the disclosure of Japanese Patent Application Laid-open No. 6-194664, a method is proposed in which a rubbing cloth worked into a cylinder is stretched over a pair of rollers, and a charge-eliminating unit and a clean unit are provided on the way between them to remove the foreign matter from the rubbing cloth. This method, however, also can not avoid the foreign matter being caused by the wear of the rubbing cloth.
In the disclosure of Japanese Patent Application Laid-open No. 8-334767, a method is proposed in which a blade for removing any adhering foreign matter or a roller coated with a sticky adhesive is brought into contact with a rubbing roller being rotated, to remove the foreign matter from the rubbing cloth. However, the foreign matter may scatter when the foreign matter is scraped off by the blade, or the sticky adhesive may adhere to the roller, and hence the foreign matter can not be avoided adhering to the substrate member.
As discussed above, when the alignment film is subjected to rubbing, the foreign matter is caused by the wear of the rubbing cloth and alignment film, and at the same time the static electricity is generated upon friction. By the action of this static electricity, the foreign matter (wear pieces and/or cut pieces) may adhere to the surface of the substrate member having been subjected to rubbing. In the conventional technique, in order to remove the foreign matter, a cleaning step is added after the rubbing so as to make the foreign matter less remain, but any satisfactory effect has not necessarily been obtained. For example, when the substrate member is cleaned in such a post-rubbing cleaning step, there is a problem that any foreign matter having adhered to areas outside the display region of the liquid-crystal display element may move to an area inside the display region, and again adhere to the alignment film surface (transfer and re-adhesion of foreign matter). Once any foreign matter has adhered to the surface of the alignment film having been subjected to rubbing treatment, the foreign matter disorders the arrangement of liquid-crystal molecules, so that non-uniform display is observed in the dynamic operating inspection carried out after the injection and sealing of liquid crystal.
Accordingly, the post-rubbing cleaning step may be omitted so that the foreign matter produced in the post-rubbing cleaning step can be stopped from adhering again. This, however, brings about a great difficulty, and it is not easy to omit the post-rubbing cleaning step. For example, there may arise problems that the foreign matter having adhered at the time of rubbing remains on the alignment film surface inside the liquid-crystal display region to cause faulty display, that any contamination due to ionic substances may occur when the alignment film is formed by coating and rubbing-treated, which also causes deterioration of display performance, and that the sealant is not well patterned in the sealant patterning step.